I think most kids that collected cards (pre-1980s) used to mark up their cards in some way. If not they at least would fill in the tick boxes on the checklists (even if they didn't have the card). One of the stupidest things I did was to mark my cards with my initial to identify my cards from my brother's. If I had been smart I would have used pencil and lightly marked it on the back, or at least did what he did and mark it small, but NO I often would mark it with a red ball point with a hugh "K" or on the back sometimes with a black magic marker. Through the years most of those "marked" cards have disappeared, but a few have survived in my collection. Fortunately they are ones that I used a smaller pen stroke.